Basics:

Sodium ions go into the cell and the electrical potential becomes more positive .

Potassium ions move out of the cell and the electrical potential becomes more negative.

A resting ion is positive on the outside where NA+ is found, the inside is negative where K+ is found.

How it works:

1. Stimulus opens NA+ channels in the cell membrane allowning NA+ to flow into the cell, this causes resting potential to rise. (depolarization)

2. K+ channels open allowing K+ to flow out of the cell. This causes the charge of the cell to become negative. (repolarization)

All or None

For a neuron to produce an action potential, the cell body has to reach a threshold voltage. Once it does it will fire completely.

http://outreach.mcb.harvard.edu/animations/actionpotential.swf <- Action potential explanation and animation.

http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swfL <- Another explanation and animation with narrarated step by step